It is known to arrange sound heads on the human body in order to carry out length measurements (Aviation, Space and Environmental Medicine, April 1982, pages 375 to 378, and May 1983, pages 458 to 463). In such an arrangement, one of the sound heads is energised electrically with an ultrasonic frequency, so that it emits ultrasonic signals, which are received by the other sound head. The spacing of the two sound heads is determined from the transmission time difference between emission and reception. The term "sound" is to cover generally all vibrations which are propagated by molecule movement in a medium, i.e. also ultrasonic waves.
It order to determine the cross-section of limbs or extremities of the human body, it is known to arrange three sound heads around the corresponding extremity, for example, the lower leg. Two of the sound heads have a relatively small spacing, which is assumed to be known. At the time of measurement, it is presupposed that these two sound heads, disposed relatively close to one another, have a prescribed constant spacing from one another, which is known and forms the basis in connection with the calculation of the area content of the triangle formed between the sound heads. The calculation of the other legs of the triangle is effected by emitting ultrasonic waves from that sound head which is opposite the short side of the triangle. The two other sound heads receive these signals. In this way, with the aid of two measurement distances and one side length of the triangle, which is assumed to be already known, the area content of the triangle is determined by calculation. This known apparatus has the disadvantage that the short side length of the triangle, which is assumed to be constant, is in actual fact not constant. On the contrary, it is the peripheral segment of the band length between the two sound heads which is constant, but not the shortest distance between these latter. It is an additional disadvantage with the known arrangement that two of the sound heads have to be disposed relatively close to one another, the consequence of which is that the triangle formed by the sound heads is, as a rule, an obtuse-angled triangle, which fills only a very small portion of the cross-sectional area of the part of the body to be measured. The measurment is therefore relatively inaccurate.